After nearly 10 years of a HD-600 I was looking forward to a 2nd hand HD-650.
I had both loved and hated my 600s. I had always found they needed to be EQed - a 2dB cut at 125Hz.
The graphs comparing the 600 and 650 showed a broad shelf some 2dB lower between 125Hz and 1kHz on the 600s. Perhaps this shelf was manifesting itself as the bass peak I heard and hated?
The differences in the treble graphs between the 600 and 650 troubled me too, what if I hated the so-called "darker" sound of the 650?!
But I was already committed, so it was too late to bale out.
My thoughts on the bass were well founded. The 600s bass shelf WAS the contributor to the bass peak I was hearing and hated.
Finally, a headphone I could listen to without searching for an EQ.
On the treble side there was a difference it was like whether I'd woken up on the left or right side of my bed. Both were different yet the same to my ears. I wasn't fazed.
Listening to rock, jazz, medieval, orchestra, solo piano or Top 40 hits, I'm glad I swapped my 600s for the 650s. Yes they cost a little more, but they ARE the more accurate pair.
To me they are, anyway!